1. Field of the Invention
The invention relates generally to a method of fabricating diode lasers whose performance is essentially unchanged over designed temperature and bias ranges, and more particularly to a diode laser of which the threshold current (Ith) and the external efficiency (xcex7ext) remain unchanged over a range of specified temperatures.
2. Description of the Related Art
Diode lasers are a mature technology and are used in many commercial applications. The cost of diode laser modules varies according to the specifications required by the application. If specifications are tight, the yield of the lasers may be low, or the laser modules may require thermal management (such as thermoelectric cooling). Both of these lead to high cost. Ideally one would like a diode laser that could be engineered so that output power is constant over temperature.
For simplicity, the laser is considered an electro-optic device where electrons are exchanged for photons (i.e., light). In an ideal laser the same amount of input current (electrons) will always produce the same amount of light (photons). For a laser used in communications this condition needs to be true for a large range of currents. More specifically, the laser needs to adhere to the linear relation: P=xcex7ext(Ixe2x88x92Ith) where P is power, I is current, and the rest are constants.
The invention provides an improved vertical cavity surface emitting laser (VCSEL) that includes an output mirror stack having a semiconductor distributed Bragg reflector and a dielectric distributed Bragg reflector, where a center wavelength of the semiconductor distributed Bragg reflector is different than a center wavelength of the dielectric distributed Bragg reflector. According to an exemplary embodiment of the invention, the center wavelength of the dielectric distributed Bragg reflector is less than the center wavelength of the semiconductor distributed Bragg reflector and less than the lasing wavelength of the VCSEL. The semiconductor distributed Bragg reflector (DBR) is made up of materials with high and low index of refractions. The reflection at any optical interface is equal to the difference between the high and low indexes of refraction divided by the sum of the same indexes. Thus, the larger the difference between the high and low indexes of refraction, the greater the reflection at the interface. This quantity, the difference divided by the sum, will be referred to as the index contrast ratio. The semiconductor distributed Bragg reflector has an index contrast ratio that changes with temperature. The dielectric distributed Bragg reflector has an index contrast ratio that changes less with temperature than that of the semiconductor distributed Bragg reflector. As the temperature increases, the lasing wavelength of the VCSEL increases and shifts away from the center wavelength of the dielectric distributed Bragg reflector such that the dielectric distributed Bragg reflector becomes less reflective.
Other features and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawing, which illustrate, by way of example, the features of the invention.